Dispersed getter elements



Dec. 4, 1956 R. J. WHELDON ET AL 2,772,771

DISPERSED GETTER ELEMENTS Filed July 14, 1955 F ig. 2

gem-r c/oscP/f MIEL ou INVENT'OR S United States Patent O DISPERSED GETTER ELEMENTS Robert Joseph Wheldon, Cricklewood, London, Arthur Everard Widdowson, and Kenneth Herbert Wilkinson, Wembley, England, assignors to The M-O Valve Company Limited, London, England Application July 14, 1955, Serial No. 522,008

Claims priority, application Great Britain July 23, 1954 Claims. (Cl. 206-.4)

This invention relates to dispersed getter elements.

Getters are commonly used for assisting in the production of a high vacuum, for example in the envelope of an electron discharge device such as a thermionic valve during the manufacture of the device. Dispersed getter elements consist at least in part of a quantity of getter material which is volatilised inside the envelope after this has been evacuated to a certain degree, the getter material during and after its volatilisation serving to absorb the gas residues left in the envelope.

According to the present invention a dispersed getter element comprises a quantity of getter material disposed in a container which is substantially completely closed a-t normal temperatures but is constructed so that when it is heated in order to volatilise the getter material an aperture suiciently large to allow the volatilised getter material to escape will be developed in its wall by virtue of differential thermal expansion of parts of the container.

Such a dispersed getter element possesses the advantage that, although the getter material may escape easily when volatilised, any residual ash left in the container after the getter material is volatilised cannot escape from the con-tainer to cause trouble in the device in which the getter element is installed; this advantage is particularly valuable in connection with getter elements intended for use in thermionic valves having small interelectrode clearances, for example those designed for usc at high frequencies.

One arrangement in accordance with the invention will now be described by way of example with reference to the `'accompanying drawings in which:

Figure l shows a vertical section of a dispersed getter element suitable for use in a thermionic v-alve designed for use at high frequencies.

Figure 2 is an exploded plane View of the element.

The getter element includes a container which is constructed from two circular cylindrical cups 1 and 2, the cup 1 being of nickel and the cup 2 being of molybdenum. The molybdenum cup 2 has an axial length of 2 millimeters whilst that of the nickel cup 1 is 3 millimeters. The external diameter of the molybdenum cup 2 at normal temperatures is 8 millimetres so that it is a close lit within the nickel cup 1, whose internal diameter at normal temperatures is 8 millimetres. The molybdenum cup 2 is disposed inside the nickel cup .1 with its rim contiguous with that of the nickel cup 1, the two cups 1 and 2 being held together by means of a nickel eyelet 3 passing through their bases, which is surrounded by a nickel washer 4 which holds the bases of the two cups 1 and 2 apart so as to form a space between them. Within this space are disposed pellets of nickel clad barium 5 and 6 which constitute the getter material.

The getter element is installed in a valve by mounting the eyelet 3 on a metal rod (not shown) and welding the eyelet to this rod. When the barium is to be volatilised, the getter element is heated by eddy current heating, and when this occurs an annular opening apff 2,772,771 ce Patented Dec. 4, 1956 pears in the wall of the container, extending between the cylindrical portions of the two cups 1 and 2, due to the thermal expansion of the nickel cup 1 being greater than that of the molybdenum cup 2. The volatilised barium is thus enabled to escape from the container. After the barium has been volatilised, the container reverts to its original closed form, so that any residual ash in the container cannot escape.

Although in the above embodiment of the invention, the differential expansion of parts ofthe container on heating arises out of the use in the container of diterent metals having differing coeicients of thermal expansion, the invention has equal application to embodiments in which this differential expansion arises out of the use in the container of portions which are maintained at differing temperatures during the volatilisation of the getter material but not necessarily consisting of dilerent metals.

We claim:

1. A dispersed getter element comprising a quantity of getter material, and an imperforate container in which the getter material is disposed and which is substantially completely closed at normal temperatures, the container being entirely constituted of parts that remain solid at a temperature above the volatilisation temperature of the getter material and including two parts which engage closely with one another at normal temperatures and which on heating the getter element to vola-tilise the getter material expand dilerentially to provide an opening between said two parts which allows the volatilised getter material to escape from the container.

2. A dispersed getter element comprising a quantity of getter material, and an imperforate container in which the getter material is disposed and which is substantially completely closed at normal temperatures, the container being entirely constructed of materials whose melting points are higher than the volatilisation temperature of the getter material and at least two of which have different coeicients of thermal expansion, and the container including two parts which engage closely with one another at normal temperatures and which on heating to said volatilisation temperature expand differentially to provide an opening between said two parts which allows the volatilised getter material to escape from the container.

3. A disposed getter element according to claim 2 in which said first metal is nickel and said second metal is molybdenum.

4. A dispersed getter element according to claim 2 in which said cups are held together by means of an eyelet passing through'two apertures formed respectively in the bases of the two cups.

5. A dispersed getter element 4comprising a rst mperforate cup of a lirst metal, a second imperforate cup of a second metal which has a lower coeicient of thermal expansion than the iirst metal, the second cup nesting inside said first cup with its base spaced from the base of the rst cup and being a close tit within the lirst'cup at normal temperatures to form an imperforate housing, and a quantity of getter material disposed in the space between the bases of the two cups, the saidA References Cited in the file of this patent UNITED STATES PATENTS 2,082,268 Varian June 1, 1937 White Nov. 22, 1938 f 

